Hmm, I've only been playing a couple of days on this site but I agree with Davetroll. It's annoying if someone doesn't log in for two or three days, which is annoying but also makes the game easier for those who border the inactive player. This is not something you can solve imo because you can't always predict if you can be online once a day. Another problem is people not using the forum in the game, or only use it very rarely which makes it hard to actually do some diplomacy in the game.
One solution would be making the forum/msg's more easy to use. I was thinking myself of some sort of a messagebox, which gets msg's from all games so you don't have to check on every game you're playing, scrolling down, opening threads over and over again, but you get your msg's all together in one msgbox. I read that the webmaster is improving on the sending of msg though, so I'll shut up about this and wait for the update of the game. :)
Another solution might be an option to choose the maximum time of 24 hours/move, 48, 72hr or even 1 week, for players who aren't active a lot. It would also give more time to check with people for your diplomacy. It can't always be done within 24hours. Negative part may be that if a move lasts one week, people will stop playing if they get annoyed/bored and you will still have the problem of inactives.

Long post, but to end this, I'm playing this great diplomacy game actively. :)